Without exception studies of overfeeding in lean volunteers have shown marked variation in the efficiency of weight gain. Within this range of variation, actual weight gain appears to be substantially less than predicted for many individuals. These findings have led to the hypothesis that man, like other animals, has the ability to dissipate energy as heat and that obesity may be due in part to a defect in this dissipative mechanism. Although it is generally accepted that thermogenic processes at rest and after exercise are normal in the obese, there are reports of differences in postprandial thermogenesis among lean and obese groups. However, the data are inconsistent and fail to discriminate between altered thermogenesis as a cause or as an effect of the obese condition. This protocol will dovetail with and involve the obese subjects recruited for an on-going NIH supported investigation of the relationship between obesity and hypertension. Additional study parameters include resting and postprandial metabolic rage, hormonal status, and substrate profile measured during each of the following phases of the existing protocol: 1) During weight maintenance before weight loss, 2) During the early phase of energy restriction, 3) During energy restriction after weight loss, and 4) During weight maintenance after weight loss. All measurements will be made at the end of 10 days of in-hospital stabilization. An additional group of subjects will be recruited as never-obese controls, with the same measurements taken after a similar period of documented weight stability. The composition of the diet will be constant in terms of energy derived from carbohydrate, protein and fat, P/S ratio, and major electrolytes, to avoid confounding effects of dissimilar antecedent dietary patterns. Results will be evaluated by two comparisons: 1) Paired changes in study parameters from the pre- to post-obese state; and 2) Group differences between post-obese and never-obese subjects. The findings of the proposed studies should serve to expand our understanding of the etiology of obesity and the role of postprandial thermogenesis in the adaptation to calorie restriction. Utilization of subjects from the on-going obesity- hypertension protocol affords tight control of the diet of the obese subjects throughout the weight maintenance and weight loss phases and greatly reduces the cost of the proposed project.